An autonomous system (AS) is a set of routers under a single technical administration, using an interior gateway protocol (IGP) and common metrics to determine how to route packets within the AS, and using an inter-AS routing protocol to determine how to route packets to other ASes. It has become common for a single AS to use several IGPs and sometimes several sets of metrics within the AS. The use of the term “Autonomous System” stresses the fact that, even when multiple IGPs and metrics are used, the administration of a first AS appears to other ASes to have a single coherent interior routing plan and presents a consistent picture of what destinations are reachable through the first AS.
The Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among ASes on the Internet. The protocol is often classified as a path vector protocol but is sometimes also classed as a distance-vector routing protocol.